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I've been learning how to make 49:1 auto transformers to run end fed half wave antennas. As I understand it, the twisted lead forms a zero voltage point which acts like a ground.

Assume I'm using coaxial feed line and that the transceiver is also grounded. The inner conductor connects to the first turn and the shield to the twisted pair.

Would it make sense to add a wire from the twisted pair to a spike in the ground ?

My reasoning is that the ground the radio is directly connected could be non-zero or RFI could get on the shield and raise the virtual ground to something not matching the radio end.

Autotransformer schematic from the web

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An end-fed acts more like a highly off-center fed dipole. Thus, the point you label "GND" is really the attachment point to the the remaining portion of the dipole, either something like a 5% length counterpoise, or sometimes the short stub of coax shield to a common-mode choke, if not the shield of your entire feed-line. If you attach a ground line of any length to this point, it would act as part of the stub portion of your off-center fed dipole, perhaps not aiming the radiation pattern in the direction you desire (as well as possibly altering the SWR and any antenna resonances).

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  • $\begingroup$ Kept changing my comment, sorry. OK I get it. I am describing the full use of the shield in the coax to the virtual null. The shield becomes the counter poise and it makes sense to use a common-mode choke. Thanks ! I'll accept after others have a chance to chime in in accordance with this groups culture :) $\endgroup$
    – wbg
    Jan 13 at 19:27
  • $\begingroup$ I've learned since this post that adding a 5% counter poise is ideal as it moves the input impedance lower as it shifts the feed point. Basically what the answer says. I'll still run with a CMC since I'm worried that my antenna could couple in unpredictable ways upsetting the balance. $\endgroup$
    – wbg
    Jan 19 at 18:48

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